Bragg grating of refractive index modulation type is formed in an optical fiber consisting of SiO.sub.2 glass with GeO.sub.2 added whose refractive index can be increased by exposure to ultraviolet radiation. Bragg grating of this type is comparably easy to be produced and excellent in function, and thus has been developed extensively as a useful part for Add/Drop filter in optical wavelength multiplexed communication, dispersion compensator and optical sensor.
A conventional waveguide type grating device comprises a cladding provided on a quartz substrate and a core formed in the cladding for an optical wave guide. The core consists of a pair of non-grating portions and a grating portion interposed between the non-grating portions. The non-grating portions have an input port and an output port at outer ends, respectively.
The core in the conventional waveguide type grating device is formed of GeO.sub.2 --SiO.sub.2 whose refractive index can be increased by exposure to ultraviolet radiation, while the cladding is free from GeO.sub.2 added. Thus, modulation of the refractive index by ultraviolet irradiation is performed exclusively within the core. In such a grating device, light is confined to be propagated through the core but diffusion of light into the cladding is not prevented successfully.
Another grating device based on the same principle as the aforesaid conventional waveguide type grating device is an optical fiber grating in which a grating is formed by irradiating the core of optical fiber to ultraviolet rays and the core so treated is arranged between a pair of non-grating portions. In this optical fiber grating, too, light diffuses into the cladding as it is propagated, thus causing increase in wavelength loss. Various attempts have been made to avoid this, among which the most common and effective way is to use a grating formed in an optical fiber having higher ratio of propagated light confined within the core, for example, multi-mode optical fiber.
This method is found to be effective to suppress the increase in wavelength loss in shorter wavelength down to a negligible extent. In practical use, however, it is necessary to couple the multi-mode optical fiber with single mode optical fibers to which an optical signal is to be inputted and outputted. The single mode optical fiber differs from the multi-mode optical fiber in mode field diameter. This difference in diameter causes a significant wavelength loss at this coupling.
In order to suppress such wavelength loss at the coupling, TEC fusing process has been used in which the core diameter of single mode optical fiber is expanded gradually by heating so as to allow coupling.
For coupling a single mode fiber with a multi-mode fiber having a grating portion, the core of single mode fiber is heated by a pair of electrodes arranged in the neighborhood of the fiber at the position of coupling to expand the diameter of the core gradually up to that of the multi-mode fiber. Thereafter, the fused part of the single mode and multi-mode optical fibers is covered with a heat-shrinking tube to protect the fused part.
In an optical fiber grating using multi-mode and single mode optical fibers, however, the necessity of TEC fusing process results in higher costs, and the use of heat-shrinking tube of 40 mm in length which is longer than the grating portion of 10 to 30 mm results in increased total length, both causing problems.
Further, in a conventional waveguide type grating device, there is another problem of increase in wavelength loss, because light diffuses into cladding as it is propagated (that is, not confined within the core), thereby progressive propagation mode is coupled partly with regressive radiation mode to cause the increase in wavelength loss.